There are many RS232 to RS485 converters available. Many will work with the Modbus protocol but there are some important facts to consider when making your selection..
1. Is your Modbus network connection "full duplex" (4-wire) or "half duplex" (2-wire)
2. Will there be multiple (more than 2) Modbus devices connected to the RS485 network.
If you are point to point with a full duplex then just about any converter will work. If you are half duplex or have multiple Modbus slaves then you need to pay attention to how the converter enables its RS485 driver. You can only have one RS485 driver on at any time so it will need to be
switched on and off when required. There are two popular ways of doing this. Use the RS232 RTS (Request to Send) signal to enable the 485 driver
or else turn the driver on automatically whenever the RS232 transmit line moves. In the first case, your Modbus software and hardware must support
the RTS control. In the second case there can be some suttle problems depending on yet other system variables.
3. Are you using Modbus RTU or Modbus ASCII
4. What baud rate are you going to use
Modbus ASCII uses a special Start of Message character to determine where a message begins. This is a colon ) character for Modbus so if the
converter gets its driver on fast enough (during the first part of the first start bit of the colon) then the receivers should see the colon and
be able to clock in the remainder of the message. If the device is a little slow or there are many devices on the line or the baud rate is very
high then the receivers may not always see the colon and will miss messages. There is another problem with this type of converter at the end
of the message if you are on a half duplex system. There is a usually a timer in the converter that keeps the driver enabled for at least one complete character time after it sees activity on the transmit line. Well, if there is a transition towards the end of the last character being sent, then the driver will be held on for at leas a character time. If the
receiving device answers the message (turns on its driver) before the transmitting device has disabled its driver, then the answer will be
garbled. Too high a baud rate and you can miss the first character in the request, and too slow a baud rate and the first character in the response is clobbered.
If you select Modbus RTU, the problem gets much worse. This is because message framing in Modbus RTU depends on being able to simulate a
synchronous message. The spec says that there must be 3.5 character time of "silence" before each message. That means that the transmitter must
first enable its driver to put the line in a "marking" state and keep it there for at least 3.5 character times before sending out its message. Converters that use transmit line activity to turn on their drivers cannot do this so you must use a converter that supports the RTS method for turning on its driver and the software will have to hold off sending messages for at least 3.5 character times.
In short, there are many things to consider depending on your specific application.
What you're asking for is really quite impossible. RS-232 and RS-485 are electrical standards (how to create a signal), while Modbus is a communications protocol (defines meaning of signals between devices). Modbus can be used on top of RS-232 or RS-485...it could be used with other schemes, too. So an RS-232 to Modbus converter doesn't exist!
Modbus is a protocol that will run on anything. All you need to do is convert the media. RS232 - RS485 interface, RS232/RS485 to ethernet interface, etc etc. Depending on the interface devices you may need to adjust the timing
characteristic and the cabling but any (99%) convetor will work.
try the site of Advantech. www.advantech.com
Check the ADAM series.I have use ADAM convertes.
It is very easy to change the setup(dip switches).
ABB Constructions SA
In some cases, I've had trouble with various RS485 transceivers which seem to have difficulty consistently recognizing the idle time between messages. One unit I have had very good luck with has been the B&B converter...